In producing oil and gas, it is often desirable to make "gravity" surveys, i.e., surveys of gravitational field measurements at various locations. Gravity information has many uses. For example, it can be used to predict the location of faults, salt domes, and other subsurface features. It can be used to locate hydrocarbon bearing strata and to locate fronts of steam or water floods.
Gravity measurements are now possible to a high degree of precision. U.S. Pat. No. 4,422,329 discloses a high precision gravity meter, currently manufactured by Delta-G Instruments, that can be used in boreholes. U.S. Pat. No. 4,722,603 discloses a high precision method of finding the vertical location of a gravity meter. U.S. Pat. Nos. 4,422,329 and 4,722,603 are both fully incorporated herein for all purposes.
The theory of the deflection of the vertical is an old part of the science of geodesy. The deflection of the vertical is the spatial angle between the direction of a plumb line (sometimes called the gravitational vertical) and the perpendicular (the normal) to the reference spheroid. This difference seldom exceeds thirty seconds of arc.
W. Torge, Methods of Measurement in Geodesy, GEODESY, Walter de Gruyter, Berlin, 1980, 254 pages, summarizes the available methods of finding deflection of the vertical. For example, it describes observations of the position of stars, use of a "photographic zenith tube," the "prism astrolabe," and "portable zenith cameras." BUMFORD, PHYSICAL GEODESY (1980) also discusses this topic.
Spatial angles, such as the deflection of the vertical, can be fully defined by two angles (e.g., .theta. and .phi., shown in FIG. 2). FIG. 2 shows a first direction 1 (such as an astronomically derived reference direction) and a second direction 2 (such as an instrument characterization direction). The spatial angles between these two directions can be expressed either within the context of a Cartesian coordinate system or a spherical coordinate system.
The deflection of the vertical is currently estimated using gravity measurements, topography, and the shape of the sea surface. Elaborate astronomical measurements can establish an estimated value using special surveying techniques. Before the present invention, there was no way of making direct measurements of deflection of the vertical using a portable device on land, or within mine shafts and tunnels, or in the borehole.